This invention relates generally to a motor protector which is to be used in a compressor or the like to be employed, for instance, in air conditioners, and more particularly to a motor protector of the type to be used inside of the motor that is to be protected.
Examples of prior art motor protectors of this type are shown in FIGS. 9(a) through 9(c). As shown in FIGS. 9(a) and 9(b), a typical motor protector 101 has a support 103 having a heater 103a disposed in the main body 102a of a casing 102 made of steel. A terminal pin 104 which has been fixed to the support member 103 extends out of a header 105 that has been provided at the opening of casing 102, with the gap between this terminal pin 104 and header 105 being sealed by a glass seal 106 and an epoxy pellet 107. A bimetal disc 108 is fixed to support member 103 by means of a slug 109, with its movable contact 110 engageable with a stationary contact 111 mounted inside of casing 102.
In applications in which a motor protector as described is to be installed in a motor compressor of the sealed type (hereafter referred to as the electromotive compressor 100), first and second electric wires 112 are connected respectively to terminal pin 104 and casing 102. Motor protector 101 is disposed inside of an insulating sleeve 113 and, as shown in FIG. 9(c), is connected to windings 114 (main winding 114a and auxiliary winding 114b). Thus, motor protector 101 is serially connected with driving circuit 130 of electromotive compressor 100 connectable to an alternating current source 115.
Bimetal disc 108 snaps from one dished configuration to an opposite dished configuration due to the generation of heat by the bimetal disc stemming from overload current or, otherwise, by an elevation of the ambient temperature within the protector including the generation of heat by heater 103a, with a consequence that the driving circuit is opened thereby preventing any possible damage from being inflicted on electromotive compressor 100.
Nevertheless, such conventional motor protectors 101 have the following problem:
In the event that an abnormal state, as described above, develops in electromotive compressor 100, electromotive compressor 100 is protected from heat generation or possible burning as the motor protector 101 repeatedly conducts and interrupts the electric current. In the case where the situation is not remedied, motor protector 101 continues its protection of electromotive compressor 100 above and beyond the designed number of cycles of its life expectancy. Under such circumstances, when motor protector 101 eventually exceeds its expected product life, melting and welding of movable contact 110 and stationary contact 111 of the bimetal disc 108 occur, thereby bringing about a state of continuous current flow. If such a state continues, winding 114 in electromotive compressor 100 will become over-heated, with a result that the electrical insulating resin of winding 114 will be melted by the heat, thereby bringing about a short-circuited state. This results in a marked increase in electric current and concomitant lowering of the resistance value of the driving circuit 130, with a resultant abnormally heated state due to a large current flowing in the inner circuit of electromotive compressor 100. As a consequence of such abnormal heating, winding 114 of electromotive compressor 100 can be burned producing carbide soot which adheres to the surface of glass 106 of the fusite pin 104, thereby bringing about a loss of electrical insulation between the fusite pin 104 and header 105.
In the worst case, tracking develops on the surface of glass 106 causing the glass to soften and melt due to the heat generated by the passage of electric current, culminating in the blow-out of fusite pin 104 which is no longer able to withstand the inner pressure of electromotive compressor 100 in some cases. In order to cope with such a problem, it is conceivable to provide a fuse, for example, for the purpose of de-energizing driving circuit 130 prior to the possible blow-out of the fusite pin. In such case, there is also a concern that the arc that is generated in connection with the melting of the fuse could ignite the gas in the electromotive compressor 100, thereby causing a possible explosion.
An object of the present invention is to provide a motor protector which solves the problem of the prior art described above. Another object of the invention is the provision of a motor protector which is capable of ultimately avoiding a dangerous situation even in the case where the contacts of a motor protector weld. Yet another object of the invention is the provision of a motor protector which is capable of preventing any possible adverse affect of an arc, generated when the driving circuit has been interrupted, on the various environments of the electromotive compressor.
Briefly stated, a motor protector made in accordance with the invention comprises a casing that can be tightly sealed, a first switch provided inside the casing that carries out the switching of a movable contact in an electric path as a snap-acting disc snaps from one dished configuration to an opposite dished configuration in response to a selected level of electric current and the ambient temperature, and a second switch which is connected in series with the first switch inside of the casing and which opens the current path when it is melted by a predetermined overflow of electric current.
In cases where the contacts of the first switch melt and weld in the conductive state, a large electric current is generated. According to the invention, however, the second switch will melt, thereby de-energizing the motor. Therefore, it becomes possible to prevent the possible burning of the electromotive compressor of the sealed type and any damage from being inflicted on the sealing terminal of the protector, thereby making it possible to prevent the possible adhesive tracking of carbide to the vicinity of the sealing terminal. As a consequence of this, it becomes possible by means of the invention to prevent the possible melting of the glass portion as induced from the tracking phenomenon caused by the loss of electrical insulating characteristics of the glass seal and prevent any possible blow-out of the fusite pin caused by the loss of holding characteristic of the glass. Further, by means of the invention, there is no adverse affect on the various environments of the electromotive compressor of the sealed type by the arc generated upon melting of the second switch since the second switch is disposed in an air-tight casing.
According to a feature of the invention, a heater generates heat in dependence on the level of electric current that flows in the electric current path so that it is possible to elevate the atmospheric temperature inside the casing by the heat generated by the heater and to adjust the characteristics of the motor protector by suitably selecting the material of the heater and the temperature responsive characteristics. According to another feature of the invention, accurate adjustment of the characteristics of the protector is enhanced when the second switch also serves as a heater and when the first and second switches are connected via a connective member extending through the support member which serves as the heater. According to yet another feature of the invention, the heater can be used to satisfactorily generate heat by arranging the first and second switches to sandwich the support member, for example, thereby making it possible to easily adjust the characteristics as desired. The first switch can be supported on the support member and the second switch can be arranged on the opposite side of the support member relative to the first switch. According to an alternative feature of the invention, the second switch can be arranged on the same side of the support member as the first switch so that it becomes possible to easily maintain the distance between the second switch and the inner wall of the casing, thereby making it possible to expand design flexibility and, at the same time, to accurately prevent a possible short-circuiting after the melting. According to yet another feature, the second switch as described above, can have a meltable portion that has been formed by reducing the cross-sectional area forming a meltable part which has a large electric resistance which melts when a selected level of electric current flows therethrough. In this case, it becomes easily possible to form the meltable portion of the second switch by providing a notch cut on the sides of the plate member. In addition, the second switch can have a plurality of meltable parts formed at prescribed locations to obtain motor protectors of various characteristics, with different melting temperatures, with the meltable parts provided in conformity with the motor to be protected, as the melting characteristics will change when a plurality of meltable parts are provided. According to a feature of the invention, the second switch can be constructed employing a cylindrical member to easily prepare the second switch and minimize the cost of metal molds and materials and which allows having the meltable parts comprise fusing material being formed and cut at a desired location and, at the same time, to obtain motor protectors of various properties by changing the size of the electric current that flows to the meltable part.